1. Field of the Invention
The present invention relates to a long-period grating device which generates mode coupling between core-mode light and cladding-mode light at a predetermined wavelength in a wavelength band in use.
2. Related Background Art
A long-period grating device is an optical device in which an area whose refractive index periodically changes in the advancing direction of light is formed in a part of an optical waveguide such as an optical fiber. Such a long-period grating device generates mode coupling between core-mode light, which propagates while being confined in a core region of the optical waveguide, and cladding-mode light, which also propagates in a cladding region without being confined in the core region, under a predetermined condition. Namely, the long-period grating device converts, of the light in a predetermined wavelength band propagating as core-mode light in the core region, only a wavelength component satisfying a mode-coupling condition into cladding-mode light, while letting the other wavelength components propagate as core-mode light. Thus, the long-period grating device acts as an optical filter (see, for example, A. M. Vengsarkar, et al., xe2x80x9cLong-period fiber gratings as band-rejection filters,xe2x80x9d OFC""95, PD4).
Letting xcex2co be the propagation constant of core-mode light, xcex2cl(m) be the propagation constant of m-th order cladding-mode light, and A be the period of refractive index modulation, the mode-coupling condition is represented by the following expression:
xcex2coxe2x88x92xcex2cl(m)=2xcfx80/xcex9.xe2x80x83xe2x80x83(1)
Also, each of propagation constants xcex2co and xcex2cl(m) is a function of wavelength xcex. When light propagates through an optical waveguide, there are a plurality of cladding-mode light components, whereby the above-mentioned mode-coupling condition is satisfied at a plurality of wavelengths. However, the wavelengths satisfying the above-mentioned mode-coupling condition are discrete and are separate from each other by several tens to several hundreds of nanometers. Therefore, the long-period grating is utilized as an optical filter which selectively attenuates or eliminates a wavelength of core-mode light satisfying the mode-coupling condition.
In order to design a long-period grating which selectively eliminates a desirable wavelength of core-mode light in a wavelength band in use, the respective values of xcex2co and xcex2cl(m) are initially determined according to the structure of the optical waveguide provided with the long-period grating, so that an appropriate value of period of refractive index modulation (grating period) xcex9 is set. Then, an amplitude mask in which chromium oxide or the like is deposited on an artificial quartz substrate at intervals of the period A and a silica-based optical waveguide whose core region is doped with Ge element are prepared, and the optical waveguide is irradiated with ultraviolet rays by way of the amplitude mask, whereby the long-period grating device is obtained.
The inventors have studied the above-mentioned conventional art and, as a result, have found the following problem.. Namely, there have been cases where the conventional long-period grating designed and manufactured as mentioned above eliminates not only the desirable wavelength of core-mode light in a wavelength band in use, but also other wavelengths of core-mode light in the wavelength band in use. When such a long-period grating device which cannot yield its designed optical characteristics eliminates not only core-mode light at a wavelength to be eliminated (designed wavelength) but also other wavelengths of core-mode light, then it adversely affects optical communications and the like.
In order to overcome problems such as one mentioned above, it is an object of the present invention to provide a long-period grating device which can eliminate only a desirable wavelength of core-mode light in a wavelength band in use.
The long-period grating device according to the present invention is an optical device, based on an optical waveguide such as an optical fiber, in which a refractive index modulation area with a period xcex9 for mode-coupling a predetermined wavelength of core-mode light to cladding-mode light under a predetermined condition in a wavelength band in use from 1525 nm to 1610 nm, preferably from 1525 nm to 1570 nm, is provided in the optical waveguide.
In the long-period grating device according to the present invention, in particular, the absolute value of a loss peak due to mode coupling concerning a refractive index modulation component with a period xcex9/(2n+1) (n=1, 2, 3, 4) is 0.2 dB or less in the above-mentioned wavelength band in use.
Since the period xcex9 is set so as to selectively attenuate or eliminate only a desirable wavelength of core-mode light in the wavelength band in use without attenuating other wavelengths of core-mode light, this long-period grating device is favorably utilized in optical communications and the like.
Specifically, letting xcex2co,1 be the propagation constant of core-mode light at a lower-limit wavelength in the wavelength band in use, xcex2co,2 be the propagation constant of core-mode light at an upper-limit wavelength in the wavelength band in use, xcex2cl,1(m) be the propagation constant of m(xe2x89xa6l+1)-th order cladding-mode light at the lower-limit wavelength, and xcex2cl,2(m) be the propagation constant of m(xe2x89xa6l+1)-th order cladding-mode light at the upper-limit wavelength, the above-mentioned period xcex9 is set so as not to exist within the range of at least (2n+1)xc3x972xcfx80/(xcex2co,1xe2x88x92xcex2cl,1(m)) but not greater than (2n+1)xc3x972xcfx80/(xcex2co,2xe2x88x92xcex2cl,2(m)).
In the parameters mentioned above, n is a positive integer of at least 1 but not greater than 4; whereas, letting xcex2co,max be the propagation constant of core-mode light at a coupling peak wavelength at which mode coupling between core-mode light and cladding-mode light is maximized in the wavelength band in use, 1 is the order of cladding-mode light having the propagation constant closest to xcex2co,maxxe2x88x929xc3x972xcfx80/xcex9 at the coupling peak wavelength.
When the period xcex9 of refractive index modulation is set as mentioned above, the long-period grating device can selectively attenuate or eliminate only a desirable wavelength of core-mode light in the above-mentioned wavelength band in use without attenuating other wavelengths of core-mode light.
Here, as is also explicitly shown in U.S. Pat. No. 5,703,978, this long-period grating is a grating which induces coupling (mode coupling) between core-mode light and cladding-mode light propagating through an optical transmission line such as an optical fiber, and is clearly distinguished from short-period gratings which reflect light centered at a predetermined wavelength. Also, in order to attain a strong power conversion from core-mode light to cladding-mode light, the grating period (pitch) of the long-period grating is set such that the optical path difference between the core-mode light and the cladding-mode light becomes 2xcfx80. Therefore, the long-period grating acts so as to couple the core-mode light to the cladding-mode light, whereby the core-mode light attenuates over a narrow band centered at a predetermined wavelength (loss peak wavelength).
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.